A zero voltage transition (ZVT) full bridge converter is a well known dc/dc switch mode converter that can be controlled such that the switching elements in the full bridge, typically mosfets or igbts, are turned on under zero voltage conditions. More particularly, and looking now at FIG. 1, there is shown a conventional zero voltage transition (ZVT) full bridge converter 1. Conventional ZVT full bridge converter 1 consists of an input capacitor 2 followed by an array of switching devices 4, 6, 8, 10 arranged in a full bridge configuration, the output of the full bridge is then connected through an optional primary inductor 12 in series with a high frequency transformer 14, the output of the high frequency transformer 14 is then rectified by diodes 16, 18, 20, 22 in either a full bridge configuration or a center tapped configuration, and the rectified output is then filtered by an output inductor 24 and a storage capacitor 26.
Looking next at FIGS. 1 and 2, the conventional ZVT full bridge converter 1 is controlled by the use of two control signals 36, 38 which control operation of switching devices 4, 6, 8, 10. Each control signal is a high frequency square wave 32/34, typically 10 KHz to 300 KHz, operating at a fixed 50% duty cycle. Power conversion is controlled by adjusting the phase angle (“phi”), 28, between the two signals. Minimum or zero power occurs when phi is 0 degrees, so that both control signals are precisely in phase and the full bridge output is zero volts.
The ZVT full bridge converter may be run in discontinuous conduction mode (DCM), continuous conduction mode (CCM) or critical conduction mode (CrCm). Discontinuous conduction mode (DCM) is a mode of operation where the main inductor current returns to zero in each switching cycle. Continuous conduction mode (CCM) is a mode of operation where the main inductor current does not return to zero in each switching cycle. Critical conduction mode (CrCM) is right at the cusp of the two aforementioned conduction modes.
The conventional ZVT full bridge converter 1 of FIG. 1 is typically used to convert from a high voltage (e.g., 100 to 400 volts dc) down to a lower output voltage (e.g., <50 volts dc), with isolation between the input and output.
The conventional ZVT full bridge converter 1 of FIG. 1 is NOT well suited to the reverse situation where the input voltage is low and the output voltage is high. This is because in this low input/high output situation, the voltage stresses on the output diodes 16, 18, 20, 22 are significant. Ringing voltages occur on the output diodes 16, 18, 20, 22 that are large and difficult to snub, forcing the circuit designer to use output diodes rated for voltages much higher than the output voltage of the converter. Furthermore, when the output voltage is high, the output inductor 24 is not well utilized, since it is located at a point in the circuit where current is relatively low.
The primary inductor 12 is considered optional in a conventional ZVT full bridge converter, and may be included if additional primary inductance is desired. The primary inductor 12 adds to the leakage inductance of the transformer 14. Leakage inductance of the transformer is frequently beneficial in the operation of the converter, and limits large current spikes from passing through the transformer, and also aids in zero voltage transition operation of the converter.
In photovoltaic (PV) array applications, it is frequently desired to convert from a low voltage (e.g., <50 volts dc) up to a higher voltage (e.g., >200 volts dc), with isolation between input and output. Photovoltaic (PV) array applications typically also call for high reliability, high efficiency and low cost.
In view of the foregoing, it will be appreciated that a conventional ZVT full bridge converter (e.g., the conventional ZVT full bridge converter 1 of FIG. 1) is not well suited for use with a photovoltaic (PV) array application, since the use of a conventional ZVT full bridge converter in a low input/high output conversion creates reliability issues for the output diodes, and results in poor inductor utilization and lower efficiency.
Thus there is a need for a new and improved zero voltage transition (ZVT) full bridge converter which allows for improved operation in applications requiring high voltage output, and particularly for use in photovoltaic (PV) array applications.